Methods and devices for galvanically isolated current measurement are known in the function and form of differential current monitoring devices for electric systems. According to VDE (Association for Electrical, Electronic and Information Technologies e.V.) regulations, it is the task of a differential current monitoring device to monitor electrical installations or circuits for the occurrence of a differential current and to report it by an alert if the differential current exceeds a predefined value. For this purpose, all active conductors of the cable outlet to be protected are guided as a primary winding through a measuring current transformer having a secondary winding. The measuring current transformer constitutes a magnetic circuit in which the magnetic field generated by the current flowing in the primary winding is guided and thus transformer-coupled to the secondary winding via a shared magnetic flux. The closed magnetic circuit can be composed of several segments of varying permeability, for example of a magnet core material of high permeability and an air gap, which always has the relative permeability μrel=1.
In a power supply system without faults, the vectorial sum of all currents, and thus the differential current, equal zero so that no voltage is induced in the secondary winding. If, however, a fault current is flowing off to earth, e.g. as a result of an insulation fault, then a differential current flows through the measuring current transformer, whose magnetic circuit, in the case of a change in time, induces a voltage on the secondary side, which can be detected and evaluated. Due to the induction principle, such an arrangement is only capable of detecting changes in time in the magnetic flux and thus only primary-side current changes causing these flux changes.
However, consumers connected to the electric system, such as electric machines which have electronic semiconductor components like diodes or thyristors in rectifiers or frequency inverters, can also cause differential currents which do not have a purely sinus-shaped temporal progression, but a pulsing one or which occur as a direct differential current. Therefore, an AC/DC sensitive current measurement should be sought for these fields of application.
From the state of the art, methods for AC/DC sensitive measurement are known which work based on the principle of magnetic compensation. Therein, two secondary windings are arranged on the current transformer core, the measuring current transformer being integrated in a push-pull oscillator as an element generating oscillations. Due to the oscillator principle, the oscillating frequency is permanently flowing through the magnetic characteristic curve of the core up to the saturation range. Thereby, direct current magnetizations are compensated, wherein the compensation current flowing through the oscillator is proportionally influenced by the alternating differential current and the direct differential current and can be evaluated. It proves disadvantageous in this method that the magnetically soft core material has to meet high requirements due to the operation of the oscillator. Additionally, especially in larger measuring current transformers, only low bandwidths can be achieved with regard to the alternating differential currents to be registered.
Other methods for AC/DC sensitive current measurement are based on compensation circuits in connection with the use of magnetic field sensors, such as Hall probes, magnetoresistive sensors, or on flux gate magnetometers. Such arrangements oftentimes only have a narrow dynamic range.
Finally, there are methods for measuring currents using shunt resistors, wherein the galvanic isolation is effected by isolation amplifiers and an evaluation of the measured values takes place by means of digital signal processing. However, the registration via shunt resistances in connection with a separate galvanic isolation can prove to be a costly solution, in particular in the case of higher supply voltages in the network to be monitored.